Television



Juny 9,1935. Q O- BROWNE 2,007,594:

I TELEVISION Filed July 11, 19:53 l

INVENTOR ATTORNEY Patented July 9, 1935 UNITED *STATESl Prrlalw'r OFFICE England, assignorutp Electric and yMusical Industries Limited, Hayes, England, a company or" lGreat Britain Application July 11,1933, serial No.y 679,892 .In Great Britain July 6, 1932 1 claim; (cl. 17e-Ley The present invention relates tortelevisionl.-

A fundamental feature of all television scanning systems is the production of relative motion .between an object and a scanning aperture'.

The terms object and scanning aperture should be given broad meanings. In the ,casel of transmitting systems, for example, the object may bepeither the actual object ofY which electrical images are to be transmitted or may be an optical image of a real object, Whilst in the case of reception the kobject is usually a viewing screen.` Similarly the aperture maybe either a real aperture organ optical image of areal aperture, such as is used in so-called spot scanning. v

In most known systems, lightwhich passes `through the scanning aperture at the transmitter is utilized `to generate picture signals in a, photo-electric device, the signals so lgenerated being amplified and transmitted to a distant receiver where they are again `amplified and used to control the amount of light which passes through a receiver scanning aperture on to a screen. Y .f Y

The neness with which the object is scanned at 4either end is determined by the ratio"` of the size of the object to the size of the scanning aperture, and it is clearly desirable that this ratio should be maintained as high as possible'. l y If, however, it be desired to make.` any change in this ratio it is clearly more convenient both at the transmitter and the receiver to alter the area of the scanning aperture rather than to alter the area of the object, a diminution of the aperture size eliecting an, increaseY inziineness and an enlargementl of the aperture leffecting a decrease in neness. y A difficulty which is common to most systems of television is that of passing. sumcient ught through the scanning aperture to give rise to an adequate picture signal in `the vscanning photoelectric cell, and oir account of this dinic'ulty it has been found desirable to set the ampl'ierat a; certain fixed maximum level of an'iplication and to ensure that the picture signalsupplied to' the amplier is always of suicient strength to override any parasitic backgroundeffect analogous toA valvelnoise or to the lwell-lnovv Johnson noise encountered in' audio-frequency ampliers and Which is due to the rsistances incorporated in the amplifying circuit.

In general the picture signal supplied tothe amplieris that generated in the scanning photoelectric cell and this in turn isa true representa-A tion of th udtuatins' in the light falling on Vthe cell. Y

controlled in such manner that the average peak value, in .other words, the average value of the envelope of the picture' signal current, taken over a few picture frames, is maintained at a sub- 'stantially constant level,v

Thus, if an alternating current ampliiier be used;- only the average amplitude of the picture signal supp-lied to' the amplifier need be mainztained constant and, if a dii-'ect current amplifier be used, the average peak Value of the picture signal supplied to the amplier must be main- .tained substantially constant.

NoW assuming that the scanning system operates inra constant manner, a change in the average' amplitdeor of the' average' peak value of Athe picture Signal can only occur as a result or a comparatively slow change, that is to say a change talking place over a few' picture frames, in the nature of the object.

Achang'e3 inthe average amplitude of the picture signal, or ofv the alternating component of the' light falling upon the scanning cell, is due to a change inv the' contrast or, more accurately, the' amplitude o'f black to white in the object (Whiclr or course may be' due to' a change in the intensity of the lightI illuminating the object) a-nd achange irr the averagev peak value of the picture signalv or of the light falling upon the cell jisdue toe;` change-in the average peak value of `the object, ythat'is to say toA a change in either the average brightness or the average amplitude vof blaakte; White in the picture or in both. A ,change inthe average peak value of the object .occurs-,Whenthe intensity' of the light projected uponthe objectfor the purposes of scanning Vchanges inintensity and also .when the averag opacity of the lmchanges; .Y

Y According to the present ,invention there? is provided a method of transmitting images of an ,object toa dista-nce in' which light modulated in accordance with thel tone valuev of the object is passed into a light sensitive device, wherein changes in average amplitude or of average peak value of the light falling upon the device, due to changes in the nature of the object, are diminished.

Other features of the invention will appear from the following description andthe appended claims.

A constructional embodiment of the invention will be described by way of example, reference being made to the accompanying drawing in which Fig. 1 illustrates diagrammatically a scanning system arranged in accordance with the present invention and adapted for use at a transmitting station, and Fig. 2 illustrates, also diagrammatically, a scanning aperture of adjustable size such as may be used in the system illustrated in Fig. l.

It will be assumed for convenience that two motion picture films are to be transmitted, the first being of high average transparency or brightness and high average amplitude of black to white, that is to say, of high average peak value, and the second film being of comparatively low average brightness and low average amplitude of black to white. It will be assumed also that in both cases the picture signals are to be amplified by means of an amplifier capable of amplifying both alternating and directeurrent.

A conception of the difference between the two films may be obtained by supposing that in place of the two films mentioned above, one film is scanned twice in succession, the first time by means of a scanning beam of high'intensity and the second time by means of a beam of low intensity. In the rst case the object scanned is of high average brightness and high average amplitude of black to white and in the second case the object is of comparatively low average brightness and low average amplitude of black to white.

The two scannings, whether of the same or of different films, are effected by means of the same apparatus.

The film I of high average peak value is flrst moved, in a direction perpendicular to the plane of the paper, past a scanning gate 2 at uniform speed whilst it is illuminated by light from an arc 3. The arc is adapted to emit light of constant intensity. The gate 2 is in the form of a slit disposed perpendicularly to the direction of motion of the film. A drum 4, which is adapted to rotate about an axis parallel to the direction of motion of a nlm l carries a plurality of plane mirrors 5 all of which are arranged symmetrically and tangentially upon its surface. 'Each mirror 5 on the drum 4 reflects an image of a strip of the film in the gate 2 over a single rectangular scanning aperture G behind which is disposed a photo-electric cell 1 associated with an amplifier 8. As the film l moves past the gate 2 and the drum rotates, images of successive strips of the lm are translated over the aperture 6 and corresponding electric picture signals are generated in the cell 1.

The arc 3 is adjusted to give its maximum output of light and the amplifier 8 ris adjusted to give maximum amplification Without there arising an appreciable parasitic background effect due either to noise generated by valves or to the Johnson noise referred to above.

The picture signals generated in this way are transmitted, together with synchronizing impulses, by means of apparatus which will be described later, and at the receiving station the picture signals are reformed into images of the picture film in any known or suitable manner.

Images of the film are also reconstituted at the transmitting station; this is achieved by means of scanning apparatus which is not shown but which may be similar to that above described working in the inverse'manner. The flneness and average peak value of the transmitted images may be judged by eye or may be measured from this reconstituted image and alterations of these features of the image are effected by altering the size of the scanning aperture S by means of a shutter 9 which forms one boundary of the aperture 6.

Measurements of the average peak value of the transmitted images may be made by applying the picture signals to a peak voltmeter. Alternatively the picture signals may be analyzed by applying them to one pair of defiecting plates of a cathode ray oscillograph, a comparatively low frequency saw-tooth wave being applied to the usual second pair of deflecting plates of the oscillograph.

Alterations in the area of the scanning aperture in the transmitting apparatus vary, in effect, the number of elemental areas into which each picture is divided for transmission. If this number is too few the flneness of scanning is too low and the definition of the reconstituted image is poor, although the amplitude of the signals generated in the photo-cell is high; if the number is too great there is a greater detail in the reconstituted image than the eye can appreciate, and the probability is that the transmission channel cannot handle such high detail without serious distortion.

Furthermore, if the fineness of scanning is increased unduly and the scanning aperture made too small, the picture signal supplied to the amplier would be too Weak to override the parasitic background effect mentioned above. In general, therefore, the detail and average peak value of the picture provide a good indication of the nature of the signal supplied to the amplifler. Initial setting of the picture signal arnplitude can thus be made from an inspection of the picture and adjustments subsequently made by reference to the peak voltmeter or cathode ray oscillograph.

The film of comparatively low average peak Value is now passed through the scanning apparatus. Compensation for its low average peak value cannot be achieved by altering the arc 3 or the amplifier 8 since these are already giving their maximum outputs, but in accordance with the present invention compensation is achieved by increasing the area of the scanning aperture 6.' The increase in area serves to decrease the fineness of' scanning and also to increase the average brightness and amplitude of the light falling upon the cell 1 and is adjusted until the picture signals developed in the photo-electric cell 1 are roughly the same as for the rst film.

The second film is thus also transmitted under the best possible conditions of average brightness and flneness of scanning, whilst the amplifiers have for the whole transmission been working at their maximum amplification. The frequency Vand phase characteristics of the amplifiers thus remain constant although variations of both average brightness and neness of scanning are achieved.

If the amplifier 8 is capable of amplifying both alternating and direct currents, as has been assumed so far, then the amplified signal provides escasa fa true Yrepresentation of the average peak va-lue of the lightfalling upn thecell y1; vvinstead,there-- fore, of varying the aperture size manually,fthe 'amplified currentV may be used to'controljautornaticallyY the amount of light falling upon the cell so ythat its average peak value is maintained `substantiallyconstant.` -v Y JvThis may also be done ifthe amplier Ilv is capable of amplifying alternating current only because in this case it is only the alternating component of the light falling uponv the cell 'I "which need be maintained constant.r 1 AIn either case, therefore, the amplified'fpicture `signal may be used to control automaticallyl the size of-the`scanning aperture. t f

The picture -signals derived from""either of the `films are therefore divided into two" chanfnels. One of the two sets of signals is fedi-to a mixingramplier VII! where the picturek signals are mixed with synchronizing signals prior tol ampliilcation in the amplifier I I and transmission'over a single channel, and. the-second :set is fedgfif necessary after further amplificationA in the amplifier I2, into` a rectifier I3. The"rectied impulses .are then arranged topperate'jjfor exlample by means of` an electromagnetic device M,

the shutter 9 which constitutes one boundary of the scanning aperture-6.

The electromagnetically operated shutter 9 is illustrated in greater detail in Eig. `2.; It :comprises a moving coil' galvanometer with avane l1 substituted for the usual indicating pointer, Ithe vane being arranged to obscure the'scanning yaperture 6 in accordance with the vInovementof ythe coil I 8. Thevane I'I is counterbalancedl by a Weight I9 and the restoring forcejofjthecoil,

fusually provided by means of'a pairof springs,

is adjusted so that the shutter has a natural :fre-

vcuencysomewhat lower than theufrequency with `which successive complete `pictures onthefufilm.` are scanned. 4The natural frequency'may be; forex- 4ample, about 8 cycles per second;k `Changes in `successive complete scans are effected,- but`V all changes in average amplitude or peak value of the light falling upon the Acell,`which areduejto changes in the nature kofthe object; of 'frequency f as lowas or lower than thenatural frequency of the shutter are automatically eliminated or at least diminished. Differences in average opacity of two films are for example compensated for in this way.

Damping means may be provided in order to prevent trains of oscillations being set up.

If desired the shutter may comprise a moving coil loudspeaker movement in which the coil operates the shutter. In this case the diaphragm of the speaker may be conveniently retained for dampingy purposes and if desired may be arranged to work in a dash-pot in order to increase the damping effect.

Alternatively two of the boundaries of the scanning aperture may be obscured by means of a light valve comprising a loop of conducting ribbon carrying the rectified picture current impulses and situated in a magnetic field. Variations in size of the scanning aperture may then be effected by passing direct current of strength depending upon the amplitude of the vpicture impulses, through the conducting loop of ribbon.

In the case when the amplier associated with the' scanning photo-electric -cell is capableof amplifying only ,alternating'currenu then compensation has to be effected only for uctuations inV the alternating component of the light falling upon the cell, and in order that the vane Il may obscurefcorrectly an increasing proportion ofV the vscanning aperture 6 with increasing picture sig- 'nal amplitude, two preliminary adjustmentsare necessary. In the rst place the correct size of aperture for a given average amplitudeof the amplied picture signals is obtained by adjusting the zero of the electromagnetically operatedsh'utter my means of the potentiometer I5, and in the second place the sensitivity of the movement of thevane I 'I' is adjusted by varying the resistanceV I 6 in shunt with the coils of the electromagnetic` device operating the vane, until Va given change in the amplified picture signal amplitude produces a correct change in aperture area. f

Having once adjusted the apparatus in this way variations in the average amplitude of black to white of twonlms or in different parts of one nlm, or in any object which may be scanned, are auto- `matically ,allowed for, and the amount of light reaching the photo-electric cell is varied, at frequencies lower than the frequency with which the object to be transmitted is completely scanned,

inversely, although not necessarily linearly, with the changes in average amplitude of the object. As shown in Fig. 1,v as the drum 4 rotatesabout its axis, images of strips of the filmare translated over the aperture 6 in the direction indicated by the arrow A. The shutter 9 is thus controlled in motion in the direction of scanning only, and the width of each line scanned therefore remains constant. If desired, however, two perpendicular sides of the aperture 6 may be constituted by shutters, the motion of one of them being controlled in the direction of scanning and the motion of the otherbeing controlled in `a directionV 'perpendicular thereto. This is of particular advantage where the changes occurring in the obworkingis convenient if it isrequired to control the picture signal amplitude to flltc'any `desired extent, but not overload, theradio transmitter."

` In other cases it may be preferable to vary the area of the aperture by controlling tw'o or more boundaries independently. One boundary may be controlled bythe mean picture signal amplitude, for example, and another by the peak picture signal amplitude.

If desired,` in the case of nlm transmission, instead of varying the area of the scanning aperture in accordancewith the variations in amplitude of black to white in the film, the area may be Varied in accordance withl the variations in opacity of the background of the film, that is to say, in accordance with those variations in opacity of the nlm due tothe developing and fixing processes which the lm has undergone. This may be effected by passing a beam of light of constant intensity through an edge portion of the film lying alongside the pictures, into a photo-electric cell. Alternatively, an auxiliary cell may be disposed so as to receive light from the whole of the scanned picture and thus register the changes in average brightness of the whole picture.

In either case the currents developed in the cell, if necessary after amplification, can be the screen, is compensated by increasing the size of the scanning aperture. This has the eiect of increasing the average amplitude and` brights ness of the reformed picture and correspondingly decreasing the fineness of scanning,

It will be apparent, however, that a change either in the average brightness or in the arnpli` tude of black to White of the object at the trans-.- mitter or a change in either of these properties of the received picture currents at the receiver, may be compensated by varying the ineen intensity of the scanning light source or by varying the size of gny obturator inserted between the ,scanning light source and the cell at the transmitter, or between the scanning light .source and the View. ing screen at the receiver. An electromagnetic device operated in the manner described above may be used ,for effecting the variations in size of such an obturator. It should be noted, however, that if either of these methods be adopted, although compensation for .ch-anges in average amplitude or peal; value is achieved the optimum relationship between the average brightness and the nneness of scanning is not maintained. Com penliation is therefore preferably effectedr by varying the lSize of the scanning apertine.

It Will be clear that varying the size of the scanning `eperture or any other obturator in the perth of the light between the .Object and the scnnnlng cell has the effect of altering both the direct and alternating components of the light falling upon the cell. lf, therefore, it be -desired to compensate for changes in the alternating ccrnponent only it is necessary to oiset .the change produced in the direct component by the change in aperture size. This may be done, -for example, by projecting an auxiliary light beam upon the cell,- The scanning aperture or other obturator is then varied in size so .es to compensate for changes in the alternating component and the intensity of the auxiliary beam varied so as' .to

compensate for tbe changes in the direct .cornponont dueto the change in aperture or obturator 1t it be desired to compensate for changes in the directcomponent; only, the size of the scan7 ning aperture or obturator is maintained constant and the intensity of the auxiliary light beam varied so as to compensate for vchanges in the direct component,

The changes in the nature of the object may be auch that the average values of the alternating and direct components of the light falling upon the cell change in opposite senses, the changes not boing connected by any simple relationship. In this case it may be desirable to compensate for the two changes separately. This may be effected by using the scanning aperture or bturotor to compensate for changes in the alter, noting component and using the auxiliary to compensate for changes in the direct compllerlt Which are due both to changes in the nature of the object and to changes in aperture or obturator In practice the aperture or obturator may be controlled yin size by means of rectified currents derived from the alternating component of the picture signal and the auxiliary beam controlled in intensity by means of currents derived from the direct component of the picture signal- 1 claim:

A television system which comprises means in.- cluding en element for scanning a subiect for television, a masking element having an aperyture .for dividing the subject into .a plurality of elementalv areas and for convertingthc light vintensities of the several elemental grens into electric signaling impulses, means for ampli? iying the produced electrical signals, a screening clement arranged to cover at predetermined time intervals vpredeterinined portions of the apertured masking element, ineens .for selecting from .the converted light velues forming the electrical signals changes .in the average peak values of the resulting .slgnels ineens yfor amplifying the se,- lccted electromagnetic mea-ns operated by said selected signals for controlling the screening element to screen predetermined portions of the element to maintain constant level of the resulting signals, and means for trans.- rnlttlng the electrical signals produced from the conversion of the optical image- CECYIL OSWALD BROWNE 

